missing BigFloat conversions

[stevengj]

This shows up when trying to use DecFP with DifferentialEquations.jl:

julia> using DifferentialEquations, DecFP
julia> f = (u,p,t) -> (p*u)
julia> prob_ode_decfplinear = ODEProblem(f,Dec128(1)/Dec128(2),(Dec128(0.0),Dec128(1.0)),Dec128(1.01))

julia> sol =solve(prob_ode_decfplinear,Tsit5())
ERROR: StackOverflowError:
Stacktrace:
 [1] Dec128(::BigFloat) at /Users/stevenj/.julia/dev/DecFP/src/DecFP.jl:117
 [2] convert(::Type{Dec128}, ::BigFloat) at ./number.jl:7
 ... (the last 2 lines are repeated 39990 more times)
 [79983] Dec128(::BigFloat) at /Users/stevenj/.julia/dev/DecFP/src/DecFP.jl:117

(Solving differential equations is probably a poor fit for decimal floating-point arithmetic, but it is a good test of genericity.)

Seems like we could do this conversion by going through strings first.